Hum reduction in amplifier networks



June 6, 1939.

L. F JONES 2,161,413

HUM REDUCTION IN AMPLIFIER NETWORKS Filed May 26, 1936 1 Hal. Y I f 1.9 l/VFII I 5 7 115@ I a 'r ou-rPu'r .35 3-9 llvPu'r 41 au'rPu'r l T r I E 4a I W 44 3nnentor Loren; F. Jon e6 Gttorneg in amplifiers.

Patented June 6, 1939 PATENT OFFICE HUM REDUCTION IN AMPLIFIER NETWORKS Loren F. Jones, Merchantville, N. J., assignor to Radio Corporation of Delaware Application May 26,

. 7 Claims.

My invention relates to amplifier networks. More specifically my invention is a method of and means for reducing hum or distortion arising within amplifier networks.

1am! aware of arrangements for reducing hum one well known method is a feed back system in which the residual hum in the out-put of an amplifier may be amplified. The residual or amplified hum is fed back to an earlier, stage of the amplifier and there induced 'to oppose the original ,hum. The residual hum is then inverselypropor'tional to the feed back amplification. If the're'sidual'hur'n is to be made "very small, the feedback amplification must be instead of correcting the difiiculty, operate to cation of the invention, and

Figure 3 is a circuit diagram illustrating an.

merely shift the oscillation or excessive regen erative point'to a diiferent frequency, generally Within the useful band. I

The present invention contemplates a method of reducing hum bymeans employing the total hum produced in the network rather than the residual hum after cancellation. This method requires a minimum. of hum amplification and therefore a negligible problem of excessive regeneration and self oscillation. Furthermore in the present'inventionordinary band pass networks may be used without deleterious effects on the amplifier network. r V

n One of the objects of my invention is to reduce the hum in the output of an amplifier network by utilizing the total hum within the network, and feeding back this hum to cancel theoutput.

Another object is to eliminate output hum by amplifying the hum to overcome losses and feeding back the amplified hum to thereby cancel the original output hum without deleterious oscillat'ion of the amplifier. V

A further object is to correct regenerative tendencies outside the useful range of the amplifierv by the use of appropriate filters in the hum feedback network. I y? e In the accompanying figures, Figure 1 is a schematic diagram of one embodiment of my invention, Figure 2 is a schematic diagram of embodiment of the'invention. I e

In Figure 1, a pair of input terminals are represented by reference numerals'l 2; The input terminals are connected through the secondaries a modifi f America, a corporation 1936, Serial No. 81,797

5, l of transformers 9, l l to the input of an amplifier 13. The output of this amplifier I3 is connected to theinput I5 of a second amplifier I1. The output of the second amplifier is represented by numerals i9, 21. A potential source of hum is represented by the block 23. It will be seen that this hum is established within the second amplifier. Hum potentials of this type may be generated by alternating currents used for heating the cathodes, by incomplete filtering in the anode current source, or similar disturbances.

The input IE to the second amplifier I1 is connected to the primary 25 of transformer I l. The output l9, 2! of the second amplifier is connected tothe primary 2? of transformer 9, The transformers 9-41 are disposed so that no coupling exists between them, and their secondary potentials of the-desired signals are opposed to each other andare adjusted to equality. Furthermore, the secondaries are connected in a high impedance input circuit; Because of the negligible coupling between input and output of the second amplifier no difficulties are experienced by the self-oscillation of this amplifier [1.

Hum potentials produced within the second amplifier appear as a difference in the form of the input and output potentials of the amplifier ll. This potential, representing hum, is fed back through the transformers 9Il to the input circuit l-2. The amount of feedback and the phase thereof may be adjusted to exactly oppose the hum created within the second amplifier l1. Normal signal potentials are not fed back due to the phase opposition of the secondaries.

The compensating hum potential now appearing across the input terminals is impressed, through transformer l l, on the input of the first amplifier in proper phase to maintain this compensating hum. The compensating hum fed through the amplifier l3 opposes the hum produced in I! and may be made to substantially cancel such hum effects. Thus an anomalous condition exists in which hum or distortion created within the second amplifier may persist within the amplifier but it is eliminated from the output terminals of the amplifier.

The stability of the second amplifier I! has already been explained. It has been found that a small gain in thefirst amplifier is sufficient to overcome the losses in the feedback circuit, and because'of the small gain and the phasal connections of the entire network, the first amplifier is not inclined toward self-oscillation. Since the operation of the device is dependent on the entire hum potential, rather than the residual hum,

minals 3|, 33 are connected to the input of an amplifier network 35 through a band pass circuit 31. The output of the amplifier network is represented by terminals 39, 4|. One of the input terminals and one of the output terminals may be grounded as shown. The other input 3| and output 39 terminals are connected by a resistor 43 of high value. The resistor 43 is arranged to establish opposing signal potentials at a tap 44 intermediate its terminals. The high impedance of this resistor offers negligible coupling between the input and output circuits of the amplifier. A resistor 45 of low value is connected between the tap 44 intermediate the ends of the high impedance resistor 43 and ground.

The low impedance resistor 45 is connected to the input of an amplifier 41 by means of a slider 49. The output of the amplifier 4'! is connected to a band pass circuit 31. This circuit 37, as previously explained, is serially included in the input to the amplifier network 35. The band pass circuit may be used to eliminate currents of frequencies which tend to establish excessive regeneration. This circuit 31 may likewise be used as a phase correcting network. In the operation of the circuit of Figure 2, it is presumed that a source of undesired hum potential exists within the amplifier network Although signal potentials impressed on'the input 3|, 33 are substantially eliminated in the resistor 45, the hum currents within the amplifier 35 will fiow through the low impedance resistor 45 to ground. The potentials developed by these ourrents are impressed on the amplifier 41, and are therein amplified just sufficiently to overcome the losses in the resistors 43, 45 and band pass circuit 31. The amplified potentials are impressed on the input to the amplifier network 35 in proper magnitude and. phase to substantially cancel the hum within the amplifier network 35. The amount of hum potential fed back may be determined by adjustment of the slider 49 or gain of the amplifier 41. The band pass circuit 37'may be used to eliminate currents of a frequency which cause excessive regeneration. This circuit 3! may also include means for correcting undesirable phase shifts.

' In Figure 3, the input circuit is represented by the transformers 5!, 53 whose windings have been serially connected as shown. The secondary 55 of input transform-er 5! is connected to the input circuit of athermionic amplifier 51. The output of this amplifier 5i may be coupled by a transformer 59, or other suitable coupling means, to the input of a second thermionic amplifier 6|. The output of the second thermionic amplifier is represented by a transformer 53. The cathodes 65, 61 may be self biased as shown. The cathodes are energized by heaters 59,. II which are energized by alternating current from a source represented as 13. It is presumed that the undesired hum originates in this source 13 which is within the amplifier network just described.

The high potential points of the first amplifier input and the second amplifier output are connected by a resistor 15. The coupling and the impedance of this resistor are in accordance with the preceding description of resistor 43. A resistor 11, whose impedance is relatively low, is connected between a point intermediate the terminals of the resistor 15 and ground. A slider 19 is connected to the resistor 11, and serially through the biasing battery 8! to the grid 83 of an amplifier 85. The cathode 81 is connected to a battery 89 and to ground. The output of this amplifier 85 is connected to a band pass circuit 9| which may include a phase correcting network. The output of the band pass circuit 9| is connected to the primary 93 of transformer 53. Inasmuch as the theory of operation is essentially the same as the foregoing circuits, it is not necessary to repeat the description.

It should be understood that my invention is not limited to any particular type of amplifier but may be applied to any amplifier network in which an undesired hum is established. For example, the foregoing circuits may be applied to audio amplifiers, power amplifiers, modulators, and radio frequency amplifiers. I have also discovered that these circuits are entirely suitable for the elimination of dist'ortional effect other than or including hum, created within an amplifier network. The phase correcting networks may be applied to any portion of the amplifying system. In similar manner, band pass filters may be used in various positions other than the one illustrated.

I claim as my invention:

1. An amplifier having input and output circuits, each of said circuits including a transformer, and included between said circuits a source of undesired disturbance, means for eliminating saidundesired disturbance from the output of said amplifier comprising a high impedance circuit connecting the secondary of the transformer in said input circuit and the secondary of the transformer in said output circuit, a second amplifier having an input and an output circuit, a connection from said high impedance circuit to the input circuit of said second amplifier, and a connectionfrom said second amplifier output circuit to the first mentioned input circuit, including means for correcting undesirable phase shift of currents flowing in either of said amplifiers so that currents may be applied to said first mentioned amplifier which are of such a phase and amplitude that. said disturbances are substantially reduced.

2. An amplifier having input andoutput circuits and included between said circuits a source of undesired disturbance, means for eliminating said undesired disturbance from the output of said amplifier comprising a high impedance circuit connecting said input and output circuits, a second amplifier having an input and an output circuit, a connection from said high impedance circuit to the input circuit of said second amplifien'and a connection from said second amplifier output circuit to the first mentioned input circuit, said last named connection including serially connected means for correcting undesirable phase shift of currents flowing in the last mentioned connection. a

3. An amplifier having input and output circuits and included between said circuits a source of undesired disturbance of certain frequencies, means for eliminating said undesired disturbance from the output of said amplifier comprising a high impedance circuit connecting said input and output circuits, a second amplifier having an input and an output circuit, a connection from said high impedance circuit to the input circuit of said second amplifier, and a connection from said second amplifier output circuit to the first mentioned input circuit, said second amplifier output circuit including a band pass filter circuit for attenuating currents of frequencies other than said certain frequencies which flow through either of said amplifiers.

4. An amplifier having input and output circuits andincluded between said circuits a source of undesired disturbance of certain frequencies,

means for eliminating said undesired disturbance from the output of said amplifier comprising a high impedance circuit connecting said input and output circuits and arranged to oppose signal potentials existing between said circuits, a second amplifier having an input and an output circuit, a connection from said high impedance circuit to the input circuit of said second amplifier, and a connection from said second amplifier output circuit to the first mentioned input circuit, said last named connection including means for correcting undesirable phase shift of currents flowing in the last mentioned connection, and further including a band pass filter circuit for attenuating currents of frequencies other than said certain frequencies which flow through the last mentioned connection.

5. The method of controlling undesirable disturbing currents of certain frequencies within a band distinct from the desired frequency band, said disturbing currents originating within an amplifier having input and output circuits, which comprises coupling said input. and output circuits, causing said disturbing currents to flow through said coupling, amplifying said disturbing currents, attenuating currents of frequencies other than said certain frequencies, and impressing the resulting currents on the input of said amplifier so that currents are established which substantially eliminate the disturbing currents from said output circuit without substantial change in the amplitude of currents of frequencies other than said certain frequencies.

6. The method of controlling undesirable disturbing currents of certain frequencies within a band distinct from the desired frequency band, said disturbing currents originating within an amplifier having input and output circuits, which comprises coupling said input and output circuits, causing said disturbing currents to flow through said coupling, amplifying said disturbing currents, adjusting the phase of said disturbing currents, attenuating currents of frequencies other than said certain frequencies, and im pressing resulting currents on the input of said amplifier so that currents are established which substantially oppose the disturbing currents from said output circuit without substantial change in the amplitude of currents of frequencies other than said certain frequencies.

7. The method of controlling undesirable disturbing currents of certain frequencies within a band distinct from the desired frequency band, said disturbing currents originating within an amplifier having input and output circuits, which comprises coupling said input and output circuits, causing said disturbing currents to flow through said coupling, amplifying said disturbing currents, correcting any undesired phase shift of said amplified currents, attenuating currents of other than said certain frequencies and impressing the resulting currents on the input of said amplifier so that currents are established which substantially oppose the disturbing currents in said amplifier.

LOREN F. JONES. 

